The main aim of this thesis was to study the effects of forest management on the sustainability of integrated timber and energy wood production in Norway spruce and Scots pine based on scenario analyses using the ecosystem model SIMA, which has been validated in great detail both in earlier studies, but also in this work (Papers II-IV). For this purpose model-based analyses were used to evaluate how the varying pre-commercial stand density and thinning regimes, nitrogen fertilization and rotation length may affect the timber and energy wood production and its profitability (NPV). Concurrently, based on the use of the LCA tool, the effects of management and energy use of biomass on the potential of forests in mitigating the CO2 emissions were studied.. The study dealt mainly with the stand-level analyses for Norway spruce and Scots pine, but also at the forest landscape level of varying age class distributions for Norway spruce. The possible abiotic and biotic risks of energy wood harvest as removal and leaching of nutrients were excluded from this study.
Furthermore, experimental data from a clonal trial of Norway spruce was analyzed in this work to determine the potential offered by different clones for above-ground biomass production (Paper I).
Regarding both Norway spruce and Scots pine, the management with higher pre-commercial stand density than that used in basic management and N fertilization clearly increased timber and energy wood production and also net present value (NPV) regardless of site fertility type and rotation length (Papers II-IV). At the same time it decreased the net CO2 emissions of the energy use of biomass (Papers III-IV). The effects of fertilization
were especially pronounced when the MT site was considered in Norway spruce and VT site in Scots pine, respectively. The differences observed between VT and MT sites in Scots pine were clearly larger than between the MT and OMT sites for Norway spruce. This result is to the effect of the clearly lower nitrogen content of soil on the VT site than on the MT and OMT sites (Kellomäki et al. 2008).
On the other hand, on the MT sites, Scots pine produced, on average, more stem wood and had higher NPV than Norway spruce when similar kind of management regimes were used in simulations (Paper II). This result is related to the faster growth of Scots pine especially in the early phase of the rotation. These findings are in line also, for example, with work by Mielikäinen (1980, 1985), who showed that Scots pine has, on average, higher annual growth compared to Norway spruce on MT site when rotation length of about 80 years were used in Finnish conditions. On medium fertile sites, higher pre-commercial stand density may be preferred for Scots pine, as it increases the quality of the lower part of the stem due to decreased growth and earlier death of branches near the stem base (e.g.
Kellomäki et al. 1999). Based on this study, the increase of pre-commercial stand density compared to that used in basic management will also simultaneously increase the growth and economic profitability of the management (Papers II-IV).
For Norway spruce grown on the OMT and MT fertile sites, the highest annual stem wood production was obtained with the shorter rotation length (40 and 60 years). The annual NPV was the highest with the rotation length of 60 years, regardless of the site fertility and the interest rate used. On average, the lowest annual net CO2 emissions were obtained on the OMT and MT sites with the rotation lengths of 80 and 100 years, respectively. However, when applying management including high pre-commercial stand density and fertilization, the difference was negligible between the rotation lengths of 60 and 80 years (Paper IV).
The positive effects of fertilization and higher pre-commercial stand density on both the production of stem wood and energy wood are also indicated by the negative CO2 balance of forest ecosystem (carbon sink) and the net CO2 emissions of the use of energy wood (Papers III-IV). The emissions were the highest, on average, without fertilization as related to lower growth. However, there were clear differences between Scots pine stands regarding the total stem wood production and the CO2 emissions on the MT and VT sites, i.e. the productivity was clearly lower and CO2 emissions higher (41%) on the VT sites.
This was opposite to Norway spruce on the MT and OMT sites, but on the latter one the productivity was slightly higher.
In this work the increase of timber and energy biomass production by fertilization clearly decreased net CO2 emissions in energy production. This is in line with the results of Sathre et al. (2010) and Eriksson et al. (2007), who found that the forest fertilization can significantly reduce the net GHG emissions and increase the availability of primary energy.
According to the results of this work, the fertilization decreased the net CO2 emissions for Norway spruce on the OMT and MT sites by 17 and 23%, respectively (compared to without fertilization). For Scots pine, the reduction was 12 and 19% on the MT and VT sites as related to the increased growth. Oren et al. (2001) have also emphasized the positive effects of nitrogen fertilization on the carbon sequestration of forest stands. This is in line with the results presented here. Over the life cycle, the net CO2 emission per unit of energy were smaller for energy biomass than those for fossil fuels; i.e. the net emission was 65-152 kg CO2 MWh-1 for Norway spruce and 78-192 kg CO2 MWh-1 for Scots pine with the rotation length of 80 years. As a comparison, the corresponding CO2 emission is 341 kg
CO2 MWh-1 for coal, if excluding emissions for the production and transportation (Statistics Finland 2005).
This work indicated that management had a clear effect on the mean C stock in the forest ecosystem. This was especially the case for the management regimes which allowed a higher tree stocking in the early phase of rotation than in basic management. The same regimes increased also the production of timber and energy wood. Thornley and Cannel (2000) and Garcia-Gonzalo et al. (2007) also earlier suggested that it might be possible to simultaneously increase the timber yield and the C storage in forests by maintaining the tree stocking higher throughout rotation compared to basic management. However, in general, the C stock is highest in the forest ecosystem if no thinning is applied over the rotation. The C stock depends also on the tree species, stand structure and properties of the site (Mäkipää et al. 1998, 1999, Pussinen et al. 2002).
In general, forest bio-energy supply chains seem to be effective in terms of energy input - output ratio. In earlier studies, the energy consumption was found to be 2-3% of produced energy and the CO2 emissions were 4-7 kg CO2 eqvMWha-1, respectively (Wihersaari and Palosuo 2000, Wihersaari 2005). This held also for this study, i.e. the energy consumption varied in the range of 2-3% of that produced based on use of energy wood (Papers III-IV).
Although, the consumption of energy in regard to harvesting, short- and long-distance transportation, chipping and fertilization depend on the management regime applied, the differences are small.
In addition to the use of proper management regimes over the rotation, also the use of the most productive genetic entries in regeneration may increase the potential of biomass recovery in integrated timber and energy wood production. This was found also in experimental data analyses of Norway spruce clones, i.e. large differences existed between clones in the dry mass production of stem wood and total above-ground production (Paper I). This is in line also with the results of Bujold et al. (1996), who suggested that such differences are typical for Norway spruce provenances. The variation in the total above-ground biomass of Norway spruce clones was, in this work (Paper I), related to the variation in the average proportion of stem wood (50-66%), branches (16-25%) and needles (17-26%). These results are in line with those by Johansson (1999), who found that in Norway spruces (about same age) the share of biomass was 15% for needles, 23% for branches and 62% for stem. On the other hand, the variability within the clones was large in this work, which probably could be related to the competition between neighboring trees.
The most productive clone (clone F430) in this work (Paper I) produced potentially up to 165 Mg ha-1 in 28 years (6 Mg ha-1 a-1), if the planting density of 2500 trees ha-1 was assumed. This is 71% above the average for all the clones included in the study, and it substantially exceeds the biomass yield of 43 Mg ha-1 typically provided by Norway spruce in final fellings (Hakkila 2004). The clone F430 showed the largest above-ground growth both in terms of stem wood and crown mass. On the other hand, also the value of harvest index was high for this clone demonstrating large allocation of growth into stem. These properties also make the clone F430 a potential candidate for the combined production of timber and energy biomass. However, these findings are based on the stands with a mixture of clones, and it remains open whether this clone was as successful if grown alone in a pure stand. The preferences of clone mixtures in stands are recommended in order to reduce the risks related to pest and insect attacks and climatic variability (Roberds and Bishir 1997, Bishir and Roberds 1999). Currently, the use of clonal material is also expensive, and it will partly negate the economic profitability of the increased production, which the use of most productive clones might provide for biomass production.
To conclude, through proper forest management timber and energy wood production can be simultaneously increased and the net CO2 emissions caused by energy wood use decreased. Increased forest growth, especially due to the fertilization and higher pre-commercial stand density in the early phase of stand development, induces most of the differences between management regimes regarding the net CO2 emissions. It seems to be possible to produce forest biomass for energy purposes with relatively low CO2 emissions by applying intensive management (especially frequent fertilization). This means that it may be valuable to evaluate the current fertilizing practices applied in Finnish forestry as nowadays fertilization is considered as most profitable in mature Norway spruce (MT sites) and Scots pine stands (VT sites) 10-15 years before final felling (Harstela 2004). In addition to fertilization, also successful regeneration (e.g. tree species/genetic entry and spacing) and suitable pre-commercial stand density and rotation length provide means to increase the production of timber and energy biomass and its profitability, but also means to increase carbon sequestration (and stocks) in forest ecosystem and decrease the CO2
emissions caused by energy wood use. In the future, the effects of rotation length on timber and energy wood production and its profitability and CO2 emissions in pure stands of Scots pine and proper management in mixtures of coniferous and broadleaves should be studied in greater detail, for example. The latter is because very often broadleaves are harvested in energy wood thinning in coniferous stands.
However, in the future studies the long-term effects of the nutrient removal in biomass harvest on the forest productivity and its environmental impacts should be assessed. This is necessary because there is evidence that the harvesting of logging residues increases the loss of nutrients, which may affect the long-term site productivity (Tamm 1969, Mälkönen 1976 , Jacobson et al. 2000, Nord-Larssen 2004). According to Jacobson et al. (2000), the whole-tree harvesting has reduced volume growth in both Scots pine and Norway spruce stands (5 and 6%) during the first 10 years after felling. On the other hand, the leaching of nutrients may also be a problem in fertilization. When compensating for the loss of nutrients in whole-tree harvest, the problem is probably the largest in the first years after fertilization (Saura et al. 1995). By using slow-release fertilizers, the environmental risks of nutrient leaching after fertilization are much lower than in the case of fast-release fertilizers (Saarsalmi and Mälkönen 2001).
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